Some electronic devices are operated by inputting various information. For example, on an electronic device such as a mobile phone, an operation key for inputting various information is arranged in its operation portion, or to a personal computer, a key board (input device) having operation keys thereon is connected. On a surface of an operation key of such a kind, various information for distinguishing one from another is indicated.
However, the operation keys of the kind are arranged in positions adjacent to one another, resulting in the occurrence of erroneous inputting in some cases. In order to reduce the erroneous inputting, various kinds of contrivances are proposed. For example, in operation keys of a mobile phone, it is proposed that the position of an operation key can be identified by bending a surface of a central portion in a convex shape (for example, Related Patent Reference 1). Moreover, it is proposed that a boundary between adjacent operation keys is formed to have a shape of an indentation and by arranging an EL (ElectroLuminescent) sheet that emits light, the position of the operation key can be easily identified by the light emitted spontaneously in the boundary region (for example, Related Patent Reference 2). Further, it is proposed that a level difference portion is formed in a boundary of operation keys adjacent thereto (for example, Related Patent Reference 3).
It is also proposed that, by making small an operation surface placed in a central portion on an upper surface side of an operation key used for press-down inputting operations, the erroneous press-down of an operation portion of adjacent operation keys can be prevented (for example, Related Patent Reference 4).
It is further proposed that, by forming a step height portion in an area surrounding an operation key to obtain a flange shape and by letting wall surfaces of the operation keys be faced towards one another, the entry of foreign matters is prevented (for example, Related Patent Reference 5).